1. Field of Invention
The invention relates to a coordinate positioning machine such as a machine tool, inspection robot, or coordinate measuring machine. Coordinate positioning machines include a table for supporting an object upon which the machine is operating, and an arm movable relative to the table, typically with three linear degrees of freedom, which carries an operating module such as a cutting tool, an inspection probe, or a welding arm, for example.
2. Description of Related Art
Conventional coordinate positioning machines support the movable arm either, in the case of a robot, by a plurality of serially mounted rotatable joints, or, in the case of a machine tool and coordinate measuring machine, on a plurality of serially mounted linear guideways. In each case the serial mounting of the movable arm results in different inertial loads on the machine when the movable arm is displaced in different directions, due to the differing number of moving machine parts which must be displaced to enable such movement. Additionally, any force applied to the movable arm, for example via the operating module, will result in bending moments being applied to at least part of the structure which supports the arm.
In an alternative form of coordinate positioning machine, the movable arm is supported by a plurality of members, each of which is connected to the mechanical earth of the machine, such as the table, for example. Machines of this type are known from e.g. International Patent Application Nos. WO91/03145 (Kearney and Trecker) and WO92/17313 (Geodetic Machines), in European Patent Application No. 534585 (Ingersoll), and U.S. Pat. No. 4,732,525, and typically include a movable arm, supported relative to a fixed, or xe2x80x9cearthedxe2x80x9d structure by means of a plurality of telescopic struts. Movement of the movable arm is achieved by extension and, where appropriate, contraction of one or more of the struts. A further type of coordinate positioning machine is shown in U.S. Pat. No. 4,976,582.
The present invention provides a coordinate positioning machine having: a fixed structure; an arm, supported for movement relative to the fixed structure, upon which an operating module may be mounted; the arm being supported relative to the fixed structure by three telescopic struts, each having a motor which is actuable to increase or decrease the length of the corresponding strut; the struts being universally pivotally connected at one end to said arm, and at the other end to said fixed structure, the arm thereby possessing three rotational degrees of freedom for any given combination of lengths of the three struts; constraining means acting between the fixed structure and the arm, for constraining movement of the arm with each of said three rotational degrees of freedom to within predetermined limits, while simultaneously permitting translation of said arm with three linear degrees of freedom, and including at least one passive device which eliminates one of said rotational degrees of freedom.
In one preferred embodiment, the constraining means is entirely passive, and constrains movement of the arm with one of said rotational degrees of freedom to within predetermined limits, while eliminating movement of the arm with the remaining two rotational degrees of freedom. In a further preferred embodiment, the constraining means is entirely passive, and eliminates movement of the arms with all three rotational degrees of freedom.
Measurement of the displacement of the arm with the available degrees of freedom may be detected, to the extent desired, in any convenient manner. When rotational movement of the arm is constrained to within predetermined limits, detection of rotational displacement may be necessary depending upon the function which the machine is desired to perform. Linear displacement may, for example, be detected by laser triangulation, by transducers provided within the struts, or by the provision of a corresponding number of unpowered, or xe2x80x9cpassivexe2x80x9d telescopic struts, universally pivotally connected to the arm and the fixed structure, and containing transducers.
One advantage of a machine according to the present invention is that of a simplified construction, due to a reduction in the number of telescopic struts employed. A further advantage relates to the comparative ease and simplicity of controlling movement of the arm in real time, due to the simple geometry of the device, i.e. movement of one plane (defined by the three points of connection of the three struts at one end) relative to another plane (defined by the three points of connection of the three struts at the other end). These advantages are not however essential for the performance of the invention, nor are they necessarily the only advantages of one or more of the embodiments described.
In an alternative embodiment two additional telescopic struts are provided, each of which is connected between a mechanical earth and a point on the movable arm remote from the mounting point of the three supporting struts, the two additional struts controlling movement of the arm about two rotational axes, thereby converting the machine to a five axis machine.
The fixed structure of the machine may be provided by a frame rigidly connected to a table of the machine from which the supporting struts are suspended.
Embodiments of the invention will w be described, by way of example, and with reference to the accompanying drawings in which:
FIG. 1 shows a plan view of a first embodiment of the present invention;
FIG. 2 shows a section on the line IIxe2x80x94II in FIG. 1;
FIG. 3 shows a detail of FIGS. 1 and 2;
FIG. 4 is a plan view of a second embodiment of the present invention;
FIG. 5 is a schematic perspective view of a detail of FIG. 4;
FIG. 6A and FIG. 6B are perspective views of a modification to the embodiment of FIGS. 4 and 5;
FIG. 7 is a plan view of a third embodiment of the present invention;
FIG. 8 is a sectional view illustrating a modification of the embodiment of FIG. 6;
FIG. 9 is a perspective view of a fourth embodiment of the present invention;
FIGS. 10A-D illustrate the operation of a first part of the constraint of FIG. 8;
FIGS. 11A-D illustrate the operation of a second part of the constraint of FIG. 8;
FIG. 12 is a perspective view of an alternative to the embodiments of FIGS. 9 to 11;
FIG. 13 is a perspective view of a fifth embodiment of the present invention; and
FIG. 14 is a plan view on XIIxe2x80x94XII in FIG. 11.